1. Technical Field
The present invention relates to a spindle motor preferable as a rotational driving device for office automation equipments like computers and peripheral devices for computers.
2. Description of the Prior Art
As shown in FIG. 11, the spindle motor used for driving magnetic disc or discs of a hard disc drive device or peripheral devices for computers includes a bearing device 43 accommodated within a cylindrical bearing holder 42 extending upwardly from a base 41, a shaft 44 to which a rotor hub 45 is secured. The bearing device 43 includes a pair of upper and lower ball bearings 46, 47. These bearings include inner rings 46a, 47a attached to the shaft 44, and outer rings 46b, 47b fit within a sleeve 48. The sleeve 48 is fit and secured to the bearing holder 42. An appropriate pre-load is applied on the inner ring 47a of the lower ball bearing 47 from below. The inner ring 47a is fixedly secured to the shaft 44 under the condition that the balls 49a, 49b are kept in contact with the inner and outer rings with reasonable pressure.
The reference numeral 50 indicates a spacer interposed between the upper and lower outer rings 46b, 47b. 
Upon rising the temperature through the operation of the motor, the components of the motor expand into different dimensions. In the case of the ball bearings 46, 47, the order of the amount of expansion in the diametrical direction is;
the outer ring greater than the inner ring greater than balls.
In other words, the spacing between inner and outer rings increased upon rising the temperature of the motor is larger than the amount of expansion of the balls so that the pressure applied by the inner and outer rings on the balls i.e. the pre-load is reduced. This causes the natural frequency upon rotating of the motor to vary, and in some cases the resonance with the other components of the equipment in which the motor is incorporated.
When the spindle motor of the prior art of the above described structure is used as a drive means for the hard disc drive device, sometimes the accuracy of writing or reading out the data is deteriorated by the vibration of the drive device caused by the resonance with the other component such as a swing arm or a casing. There is also a possibility for generating noises caused by the vibration, and reducing the quietness of the drive device.
Provided that the difference of the amount of the expansion of the inner and outer rings is increased further, a clearance is defined between the balls and the inner and outer rings, and the rotational run out of the rotor hub and the run out of the surface of the magnetic disc or discs are generated. This will deteriorate the reliability of the hard disc drive device.
Although the balls are usually formed of steel material, ceramic material may also be used for enhancing the durability thereof. In such a case, the above mentioned problem caused by the difference of the amount of thermal expansion between components will become more serious, since the amount of thermal expansion of the ceramic material is further lower than that of the steel material.
Accordingly the object of the present invention is to provide a spindle motor wherein the inner and outer rings are in constant contact with the balls by applying appropriate pre-load thereto even if the components of the motor expand by rising the temperature of the motor. The purpose of the present invention is to provide a spindle motor of high rotational accuracy wherein the variation of the resonant frequency or the rotational run out caused by the variation of the temperature are reduced.
In order to achieve the purpose of the present invention, a spindle motor in accordance with the first aspect of the present invention comprises;
a double row bearing device including a shaft, a sleeve surrounding the shaft, and a pair of upper and lower rows of balls,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve within a vertical bore of the holder, and
a rotor hub mounted on the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
a reduced inner diameter portion of an inner diameter smaller than the outer diameter of the sleeve is formed on the inner surface of the bore of the bearing holder between the upper and lower larger inner diameter portions to urge the sleeve inwardly by the reduced inner diameter portion to deform it elastically to form a squeezed portion protruding inwardly of the sleeve between the upper and lower rows of balls.
A spindle motor in accordance with the second aspect of the present invention comprises;
a double row bearing device including a shaft, a sleeve surrounding the shaft, and a pair of upper and lower rows of ball bearings interposed between the shaft and the sleeve, each ball bearing has an inner ring and an outer ring and balls interposed therebetween,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve of the double row bearing device within a vertical bore of the holder, and
a rotor hub mounted on the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
a reduced inner diameter portion of an inner diameter smaller than the outer diameter of the sleeve is formed on the inner surface of the bore of the bearing holder between the upper and lower larger inner diameter portions to urge the sleeve inwardly by the reduced inner diameter portion to deform it elastically to form a squeezed portion protruding inwardly of the sleeve between the upper and lower outer rings.
A spindle motor in accordance with the third aspect of the present invention comprises;
a stepped shaft having a larger diameter shaft portion and a reduced diameter shaft portion,
a compound bearing device including;
a sleeve surrounding the stepped shaft and including a pair of upper and lower parallel outer raceways or grooves,
balls of the upper row disposed between an inner raceway or groove formed directly on the outer periphery of the larger diameter shaft portion of the stepped shaft and the upper row of outer raceway formed on the inner peripheral surface of the sleeve, and
balls of the lower row disposed between an inner raceway formed on an inner ring fit around the reduced diameter shaft portion of the stepped shaft and the lower row of outer raceway formed on the inner peripheral surface of the sleeve,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve of the compound bearing device within a vertical bore of the holder, and
a rotor hub mounted on the larger diameter shaft portion of the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
a reduced inner diameter portion of an inner diameter smaller than the outer diameter of the sleeve is formed on the inner surface of the bore of the bearing holder between the upper and lower larger inner diameter portions to urge the sleeve inwardly by the reduced inner diameter portion to deform it elastically to form a squeezed portion protruding inwardly of the sleeve between the upper and lower outer raceways.
A spindle motor in accordance with the fourth aspect of the present invention comprises;
a spindle motor comprising
a double row bearing device including a shaft, a sleeve surrounding the shaft, and a pair of upper and lower rows of balls,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve within a vertical bore of the holder, and
a rotor hub mounted on the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
an outer peripheral groove formed on the sleeve, and
a squeezing ring having an inner diameter smaller than the outer diameter of the outer peripheral groove, wherein
the squeezing ring is fit over the outer peripheral groove to elastically deform the sleeve inwardly to form a squeezed portion protruding inwardly between the upper and lower row of balls.
A spindle motor in accordance with the fifth aspect of the present invention comprises;
a spindle motor comprising
a double row bearing device including a shaft, a sleeve surrounding the shaft, and a pair of upper and lower rows of ball bearings interposed between the shaft and the sleeve, each ball bearing has an inner ring and outer ring and balls interposed therebetween,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve of the double row bearing device within a vertical bore of the holder, and
a rotor hub mounted on the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
an outer peripheral groove formed on the sleeve, and
a squeezing ring having an inner diameter smaller than the outer diameter of the outer peripheral groove, wherein
the squeezing ring is fit over the outer peripheral groove to elastically deform the sleeve inwardly to form a squeezed portion protruding inwardly between the upper and lower row of outer rings.
A spindle motor in accordance with the sixth aspect of the present invention comprises;
a spindle motor comprising
a stepped shaft including a larger diameter shaft portion and a reduced diameter shaft portion,
a compound bearing device including,
a sleeve surrounding the stepped shaft and including a pair of upper and lower parallel outer raceways or grooves,
balls of the upper row disposed between an inner raceway or groove formed directly on the outer periphery of the larger diameter shaft portion of the stepped shaft and the upper row of outer raceway formed on the inner peripheral surface of the sleeve, and
balls of the lower row disposed between an inner raceway formed on an inner ring fit around the reduced diameter shaft portion of the stepped shaft and the lower row of outer raceway formed on the inner peripheral surface of the sleeve,
a cylindrical bearing holder extending upwardly from a base of the spindle motor for accommodating the sleeve of the compound bearing device within a vertical bore of the holder, and
a rotor hub mounted on the larger diameter shaft portion of the shaft for journaling the rotor hub rotatably on the base, the spindle motor is characterized in that
an outer peripheral groove formed on the sleeve, and
a squeezing ring having an inner diameter smaller than the outer diameter of the outer peripheral groove, wherein
the squeezing ring is fit over the outer peripheral groove to elastically deform the sleeve inwardly to form a squeezed portion protruding inwardly between the upper and lower row of outer raceways.
A spindle motor in accordance with the seventh aspect of the present invention comprises;
the above described spindle motor of the second or fifth aspect wherein peripheral grooves of substantially the same width as that of the outer raceway formed on the inner peripheral surface of the outer ring are formed at positions of the inner peripheral surface of the sleeve against which the outer ring of each ball bearing abut.
A spindle motor in accordance with the eighth aspect of the present invention comprises;
one of any spindle motors of the first to sixth aspect wherein the shaft and the rotor hub are formed integrally with each other by employing the same material.
A spindle motor in accordance with the ninth aspect of the present invention comprises;
one of any spindle motors of the first to sixth aspect wherein the balls are formed of ceramic material.
A spindle motor in accordance with the tenth aspect of the present invention comprises;
the third or sixth spindle motor wherein the outer diameter of the inner ring is identical with that of the larger diameter shaft portion of the stepped shaft, and the balls of the upper row are identical in their d with that of the balls of the lower row.